Cathode ray tube



May 7,1963 N.H.LEHRER 3,089,055

CATHODE RAY TUBE Filed Feb. 8, 1960 5 Sheets-Sheet 1 Q *Qns kann" fammi May 7, 1963 Filed Feb. 8, 1960 N. H. LEHRER CATHODE RAY TUBE 3 Sheets-Sheet 2 /a, aaa

May 7 1953 N. H. LEHRER 3,089,055

CATHODE RAY TUBE Filed Feb. 8, 1960 3 Sheets-Sheet 5 77h04/ 00,1//5/4//70/1/40 044.4;- 7'04/0* .17000007005 .5f/503W Wwf/M0 .m500 l/.f ,m 70,000 fari/vra; 010f10z0ff in? United States Patent G 3,089,055 CATHGDE RAY TUBE Norman H. Lehrer, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed- Feb. 8, 1960, Ser. No. 7,163 7 Claims. (Cl. 315-12) This invention relates to a half-tone visual display storage tube and, more particularly, to a cathode ray tube incorporating a storage screen having secondary electron emission and electron bombardment induced conductivity characteristics for presenting non-stored and stored displays, non-destructive read-out and apparatus which utilizes the bombardment induced conductivity characteristics to selectively erase the stored display.

'Ihis application is a continuation in part of 4an application :for patent entitled Cathode Ray Tube, Serial No. 795,727, filed February 26, 1959, now abandoned, and `assigned to the same assignee as is the present case.

There are two well known methods of achieving equilibrium spot erasure in half-tone visual display storage tubes. First, a method known as high energy spot erasure at the second cross-over employs a high energy .gun with its cathode held negative with respect to the storage surface by a voltage which is equal to the sum of the second cross-over and storage surface cutoff potentials relative to the potential of 4the fiood gun cathode. In operation, any area of the target which 4is more positive than cutoff potential, i.e., any .area that is at -a potential that is beyond the second crossover, is driven negatively -to cutoff at the second crossover potential by the action of the erasing beam. A disadvantage with this method is that the second crossover point may vary by -as much as several hundred volts for any given target whereby the resulting nonuniformities in erasure cause variations in writing speed and brightness. These variations make this method impractical. Also, the second crossover potentials of some materials used in storage tubes may be of the order of 10,000 lvolts which introduces problems of insulation and deflection.

A second method of achieving spot erasure is known as low energy spot erasure below the first crossover po tential Iand employs an erase gun with its cathode held at the cutoff potential of the storage surface which surface is, in turn, never charged to potentials which exceed the first crossover potential of the storage dielectric relative to the erase gun cathode potential. Thus, in operation, any area of the target which is positive but which does not exceed the first crossover relative to the erase gun cathode is charged negatively to cutoff potential by the action of the erasing beam. A disadvantage with this latter method which limits its practicabili-ty is that the energy of the beam is sufficiently low to make focusing of the beam extremely diicult for any reasonable current value thereby causing a loss in resolution. In addition, the space charge limitations encountered in low energy beams does not permit appreciable current densities which result in extremely slow erasure.

It is therefore an object of the present invention to provide a storage tube incorporating an improved apparatus for effecting spot erasure of the storage surface.

An additional object of the present invention is to provide a storage tub-e incorporating an improved apparatus for effecting non-destructive read-out of the storage surtace.

Another object of the present invention is to provide a half-tone visual display storage tube incorporating a storage screen capable of being charged by secondary electron emission and capable of being erased to a determinable cutoff potential by means of bombardment induced conductivity.

Still another object of the present invention is to provide a half-tone visual display storage tube incorporating a storage screen wherein induced conductivity is achieved therethrough with a sufiiciently small potential drop there- `across as to be compatible with the visual presentation of the charge pattern thereon.

A further object of the present invention is to provide a half-tone visual display storage tube incorporating a storage screen including an insulative material having a secondary electron emission ratio greater than unity and exhibiting electron bombardment induced conductivity characteristics throughout overlapping ranges of electron energy levels.

A stil-l further object of the present invention is to provide a half-tone visual display storage tube incorporating a storage screen having a storage dielectric constituting a thin film of cubic zinc sulfide.

An additional object of the present invention is to provide a half-tone visual display storage tube incorporating a storage screen having a storage dielectric constituting a thin film of cubic zinc sulfide disposed on a substrate also having a cubic lattice structure.

An additional further object of the invention is to provide a visual display storage tube of the type described above wherein the writing speed is enhanced by means of a thin film of magnesium uoride disposed over the storage surface dielectric.

According to the present invention, high energy Writing and erase guns, together with Ia viewing gun, are disposed within an evacuated envelope in a manner to illuminate a storage screen which, in turn, is disposed adjacent to and coextensive with an aluminized viewing screen. The storf age screen is fabricated from an insulative material that exhibits a secondary electron emission ratio greater than unity and bombardment induced conductivity characteristics throughout a common range of electron energy levels so that at `one predetermined energy level the charging effected by secondary electron emission is balanced by the discharging effected by bombardment induced conductivity thus making non-destructive readout of the storage surface possible. Further, the bombardment in- -duced conductivity characteristics must exist with a total potential drop of only 7-10 volts across the layer of insulative material that provides the storage surface in order to be compatible with apparatus for visually displaying the charge pattern.

This storage screen is realized by producing a thin film of cubic zinc sulfide on a conductive substrate which preferably is also constituted from .a material of the cubic lattice type. It has been found that of the three types of Zinc sulfide, only cubic zinc sulfide possesses the desired properties suitable for use in the .present invention. Amorphous zinc sulfide was found not to possess bombardment induced conductivity characteristics and hexagonal zinc sulfide, although having bombardment induced conductivity characteristics, has a resistivity that is sufficiently low to make it unusable for charge storage purposes. Cubic zinc sulfide cannot normally be obtained by conventional evaporative processes.

In the present case, a writing beam having of the order of 2 kilovolt energy, is employed to effect writing on the screen by secondary electron emission. Simultaneously with the writing process, spot erasure is acomplished by employing an erase beam of the order of 7 kilovolt energy. It is necessary that this latter energy level be less than the -second cross-over potential of the secondary electron emission characteristic of the Ilayer of insulative material so that the eras-ing will be to a uniform determinable potential level, as will 'be hereinafter explained. Concurrently with the foregoing processes,

the charge pattern on the storage screen may be viewed on the viewing screen by means of the fiood beam of electrons produced the viewing gun. In addition to the above, one of lthe writing or erase guns or an additional electron gun may be operated at an energy level intermediate the write and erase energy levels to produce a visual display without affecting the charge pattern on the storage screen. Also, non-destructive read-out may be achieved with an electron beam of this latter energy level by `apparatus for developing a signal representative of the secondary electrons collected during this mode of operation.

The above-mentioned and other features and objects of -this invention and lthe manner of obtaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, whrein:

FIG. 1 is a cross-sectional schematic View of the directview visual displ-ay storage tube of the present invention;

FIGS. 2 `and 3 are an enlarged cut-away perspective and an enlarged cross-sectional view, respectively, of the storage screen in the device of FIG. l; and

FIGS. 4-7 show performance characteristics of the device of FIG. l.

Referring now to the drawings, FIG. l shows a halftone visual display storage tube in accordance with the present invention. This tube comprises an evacuated envelope which includes `a comparatively large cylindrical section 11 having a `face plate 12 at the right extremity as view in the drawing, and an axially aligned neck portion 13 at the left extremity thereof. The neck portion 13 houses a writing gun `14 and an erase gun 15 for producing, respectively, write and erase electron beams of elemental cross-sectional area, horizontal defiecting plates 16 and vertical deflecting plates 17 for controlling the deflection of the write beam, ho-rizontal defiecting plates 18 and vertical defiecting plates 19 for controlling the deflection of the erase beam, and a viewing gun 20 for producing a viewing beam of fiood electrons. The write and erase guns 14, 15 may, of course be niagnctically focused and deflected, in which case, however, separate neck portions may be desirable.

On the inner surface of the face plate 12 opposite the write and erase guns `14, 15 and the viewing gun 20, there is disposed a viewing screen 22 which includes a phosphor screen 23 covered with a thin film of aluminum 24. Adjacent to and coextensive with the viewing screen 22, there is disposed, in the order named, a storage screen 25 and its associated collector grid 26. Referring to FIGS. 2 and 3, there is shown an enlarged perspective portion and an enlarged cross-sectional portion, respectively, of the storage screen 25. Referring to these figures, storage screen 25 includes an electroformed nickel mesh 28 having from 100 to 400 meshes p-er finch and preferably of the order of 250 meshes per inch, a thickness of the order from 1 to 2 mils and an overall transparency of the order of 60%. When nickel is used for the mesh 28, it is desirable to plate it with a thin film of rhodium to prevent chemical reaction with the storage surface material. On Ithe side of the electroformed nickel mesh `28 facing the Write and erase guns 14, 15 and viewing gun 20, there is disposed a thin layer 29 of insulative material which exhibits a secondary electron emission ratio greater than unity and bombardment induced conductivity characteristics throughout overlapping ranges of electron energies. As is generally known, the secondary electron emission ratio is greater than unity from the first to the second cross-over potentials, the second cross-over potential being the higher potential level which is sometimes otherwise known as the sticking potential. In accordance with the invention, the layer 29 preferably constitutes a coating of cubic Zinc sulfide disposed coextensive with the meshes of electroformed nickel mesh and having a -thickness of the order of 0.65

micron. This layer 29 of cubic zinc sulfide is disposed on the electroformed nickel mesh 28 by an evaporation process which process may be performed in a conventional manner. In order to convert all of the zinc sulfide to the cubic lattice form, however, the mesh 28 is first etched in an acid bath to expose its lattice structure. Subsequent to the evaporative process, it was found that raging the storage screen 25 in a dark place either in a vacuum or at atmospheric pressure and at a temperature of from to 80 F. `for a period of approximately one month converted all of the zinc sulfide to the cubic lattice type. There are also other methods for producing cubic zinc sulfide which are known to the art. In the event that it is desired to enhance the secondary electron emission characteristics of the storage screen 25, a thin film 30 of magnesium fiuoride of the order of 500 Angstroms in thickness is evaporated over the layer 29 of zinc sulfide. In general, the then film 30 of magnesium uoride should be as thin as possible consistent with preserving the high secondary electron emission characteristics of the magnesium fluoride so as to allow a high energy beam of electrons to penetrate therethrough into the layer 29 of cubic zinc sulfide to raise electrons therein to the conduction energy level. Thicknesses in both cases may be determined by the use of an interferometer. Lastly, a thin film of gold is evaporated on the side of mesh 28 opposite the side on which layer 29 is disposed so as to cover any dielectric particles which may have been inadvertently deposited on this side.

`Referring again to FIG. l, the collector grid 26 is provided by a conductive screen having a transparency that is preferably of the order of 60%, which screen is supported about its periphery by an annular ring 32. Further, an annular electrode 33 or can is juxtaposed to the annular ring 32 of collector grid 26 and extends away from base plate 12 for a distance of several inches, the exact distance depending on the size of the tube. During operation, the viewing screen 22 is maintained at a potential of the order of 6,000 volts positive with respect to ground by a suitable connection from the aluminum film 24 to the positive terminal of a battery 34, the negative terminal lof which is referenced to ground. Also, the electroformed nickel mesh 28 of storage screen 25 and the collector grid 26 are maintained at potentials of -9 volts and |120 volts relative to ground, respectively. This may be accomplished, for example, by a connection from the annular support ring 32 of collector grid 26 through a load resistor 35 to the positive terminal of a battery 36, an intermediate terminal of which is referenced to ground. A potentiometer 37 is connected across the terminals of battery 36, `and an adjustable tap 38 is connected therefrom to the nickel mesh 28 of the storage screen 25 and set to provide the desired voltage. Also, the video read-out developed across the resistor 35 is made available by connecting a capacitor 39 directly to the collector grid 26. Next, the annular electrode 33 -is maintained at a potential of the order of +20 volts positive with respect to ground by means of a connection therefrom to an appropriate intermediate tap of lbattery 36.

Lastly, equal potential regions are maintained throughout the neck portion 13 and intermediate the annular electrode 33 and the write and erase guns 14 and viewing gun 20 by conductive layers 40, 41 disposed, respectively, about the inner periphery of neck portion 13 coextensive with the guns 14, 15, and about the inner periphery of the cylindrical portion 11. During operation, conductive layer 40 is maintained at a potential of the order of 100 volts positive with respect to ground by a suitable connection therefrom to the positive terminal of a battery 42, the negative terminal of which is referenced to ground. Also, the conductive layer 41 is maintained at a potential of the order of 5 volts positive with respect to ground by a connection therefrom to an appropriate intermediate tap of battery 42.

As previously specified, neck portion 13 of evacuated envelope houses write and erase guns 14, 15 and viewing gun 20 which are of conventional construction. The electron writing gun 14 includes a cathode 46 and an intensity grid 47. The cathode 46 of gun 14 is maintained at a potential of the order of -2000 Volts with respect to ground by means of a connection therefrom to the negative terminal of a battery 48, the positive terminal of which is referenced to ground. As will be explained, there is a predeterrninable electron energy level at which the rate of charge of the storage surface by secondary electron emission is exactly counter-balanced by the rate of discharge by bombardment induced conductivity resulting in zero net charge on the storage surface by electrons incident thereon at this predeterminable energy level. In the operation of the present device it is necessary that the electron energy level of the write beam be less than this predeterminable energy level and still remain within the range of potential levels wherein the secondary electron emission of the storage surface is greater than unity, i.e., above the first cross-over potential and less than the aforementioned predeterminable potential level. Further, the intensity grid 47 of Write gun 14 is maintained at a quiescent potential of the order of 75 volts negative with respect to the potential of cathode 46 by means of a connection therefrom through a load resistor 50 to an appropriate intermediate terminal of the battery 48. Means for modulating the intensity of the electron write beam is provided by a connection from an input terminal 53 through a capacitor 54 and across the load resistor 50 to the intensity grid 47 of write gun 14. The write beam produced by electron writing gun 14 is scanned over the storage screen 25 in a desired manner by means of horizontal and vertical deilection voltages generated by horizontal and vertical deflection voltage generators 56, 58, respectively. The horizontal deflection signals are applied to the horizontal deflecting plates 16 by means of appropriate connections thereto. Similarly, the vertical deflection signals are applied to vertical deecting plates 17 by means of appropriate connections thereto from vertical deflection voltage generator 58. The horizontal and vertical deflection plates 16, 17 are maintained at a quiescent potential of the order of +100 volts with respect to ground which is the same as the potential applied to conductive layer 40.

Similar to the write gun 14, erase gun includes a cathode 66 and an intensity grid I67. The cathode 66 of gun 15 is maintained at a potential of the order of -7000 volts with respect to ground. In the operation of the disclosed tube, it is necessary that the electron energy level of the erase beam be greater than the aforementioned predeterminable energy level and, in order to achieve maximum uniformity of the erase potential, less than the second cross-over potential of the secondary electron emission characteristic of the storage surface material. In the case of cubic zinc sulfide, the second cross-over of the secondary electron emission characteristic occurs in the range of from 8000 to 10,000 volts which is well above the 7000 volts energy level of the erase beam. The erase gun 15 is normally maintained at this energy level by connecting cathode 66 to the negative terminal of the battery 68, the positive terminal of which is connected through a switch 69 to the negative terminal of a battery 70, the positive terminal of which is, in turn, connected to ground. Alternatively, as Will hereinafter be explained, switch 69 may be connected to a terminal 71, which terminal is connected to ground. When switch 69 is connected to terminal 71, the battery 70 is bypassed whereby the cathode 66 is maintained at a potential of 4400 volts with respect to ground which will be hereinafter shown as being the aforementioned predetermined potential level. When switch 69 is connected to terminal 71, the gun 15 will write directly on the viewing scren 22 without affecting the charge pattern on the storage screen 25. Also, if a charge pattern is on the storage screen 25 during this time, the secondary electrons collected will be representative of this charge pattern. These secondary electrons will develop a read-out signal across the resistor 35 which read-out signal is available at the capacitor 39. As in the case of the Write beam, the erase beam produced by electron writing gun 15 is scanned over the storage screen 25 in a desired manner by means of horizontal and vertical deflection voltages generated by horizontal and vertical deflection voltage generators 74, 76, respectively. The horizontal and vertical deflection signals generated by deflection voltage generators 74, 76 are applied to the horizontal and vertical deilecting plates 18, 19, respectively, by means of appropriate connections thereto. Also, the horizontal and vertical detlecting plates 18, 19 are maintained at a quiescent potential of the order of +100 volts with respect to ground.

A point source of llood electrons is provided by the llood gun 20 which is disposed along the longitudinal axis of the cylindrical portion 11 of the evaluated envelope 10 at the left extremity thereof, as viewed in the drawing. Flood gun 20 includes a cathode 80 and an intensity electrode `81 which encloses the cathode 80 except for a small circular aperture 82 disposed over the central portion of the cathode 80, and an annular electrode 84 disposed adjacent to the periphery of intensity electrode 81, as shown in the drawing, and concentrically about the circular aperture `82. In operation, cathode of ilood gun 20 is referenced to ground by means of a connection therefrom to ground. Further, the intensity electrode 81 and the annular electrode 84 are maintained, respectively, at potentials of the order of -20 and +100 volts with respect to ground by means of connections therefrom to adjustable taps 87, 88 of a potentiometer 90. The potentiometer 90, in turn, is connected across the positive and negative terminals of a battery 92 which has an intermediate terminal thereof referenced to ground.

Prior to explaining the operation of the device of the present invention, reference is made to FIGS. 4--7 and, in particular, to FIG. 4, which shows the Write-erase characteristics of the storage screen 25 of the device of FIG. 1 for an electron beam of 0.020 inch spot size having a progressively increasing energy level and a constant current of 30 microamperes. In p-articular, the -abscissa of the characteristics of FIG. 4, proceeding from left to right, as shown in the drawing, there is indicated the progressively increasing energy levels for electron beams which scan the storage screen 25. The ordinate, on the other hand, shows the writing speed of the electron beam on storage screen 25. A positive writing speed at a particular energy level indicates that 'an electron beam at that energy level charges the storage surface of storage screen 25; Whereas, a negative writing speed indicates that the storage surface of screen 25 is discharged or erased by the beam. In particular, the solid line 94 illustrates the characteristic of the storage screen 25 with only the layer 29 of cubic zinc sulfide disposed uniformly over the electroformed nickel mesh 28, in the hereinbefore described manner, to a depth of the order of 0.65 micron. As is evident from the drawing, an electron beam having an energy level of approximately 2 kilovolts can write `a-t a speed of the order of 10,000 inch-volts per second on the storage `screen 25. An increase in the energy level of the beam effects a decrease in the possible writirng speed until at yan energy level of approximately 4.4 kilovolts the writing speed becomes substantially equal to zero inch-volts per second. This energy level of 4.4 kilovolts in this case is the aforementioned predeterrn-inable electron energy level. A further in-` 7 mesh 28 to the depth of 0.65 micron, which layer, in turn, is covered with the extremely thin layer 30 of magnesium uoride. This latter layer 30 is of the order o-f 500 Angstroms thick. The characteristics 94, 96 are with an average total negative voltage drop across the dielectric layers 29 and 30 of the order of 7 volts.

Referring now to FIG. 5, there is shown the writing or erasing speed of the write and erase electron beams for various potential differences across the storage dielectric layer 29 of storage screen 25 in the device of FIG. l. In particular, the abscissa is calibrated in terms of potential drops across the layer 29 and the ordinate is calibrated in Writing speed, the negative writing speeds indieating, of course, that the storage surface of storage screen 25 is being discharged. Line 98 shows the variation in the writing speed of the write beam produced by write electron gun 14 when the current is 30 microamperes, the spot size on the storage screen 25 is 0.020 inch and the energy level is 2 kilovolts. Line 100, on the other hand, shows the write-erase characteristics of the erase beam produced by the erase gun 15 when the energy level is 7 kilovolts, the spot size on the storage screen 25 is 0.020 inch and the current is 30 microamperes. In the latter case, it is noted that if the potential drops are more negative than 3.6 volts, the erase beam discharges the storage surface as shown by line 100 in the drawing. This latter point at which the erase beam produces zero charging of the storage surface is important in determining the operating potentials of the disclosed tube, as will be hereinafter described. Lastly, in order to distinguish the write-erase beams of electrons guns 14, 15 from those of a conventional half-tone storage tube, the top horizontal line illustrates the writing speed for Ka 5 kilovolt beam in a typical half-tone storage tube. As is evident from the drawing, the writing speed of the 5 kilovolt beam is completely independent of the potential drop across the storage layer and remains constant at a normalized writing speed 76,000 inch-volts per second which would indicate that there is no bombardment induced conductivity taking place. 'Il-iat is, the writing is accomplished solely by secondary electron emission.

Referring now to FIG. 6, there is shown the brightness transfer characteristics for the disclosed storage tube described in connection with FIG. `1 for various potentials applied to nickel mesh 28 of storage screen 25 without the thin tlm 30 of magnesium fluoride. In this figure the abscissa is calibrated in storage surface potential relative to the potential of viewing gun cathode 80 and the ordinate in percentage of relative brightness. It is evident that any potential on the storage surface of screen 25 that is positive relative to the potential of viewing gun cathode 80 will immediately commence to be discharged to zero volts by the action of the flood electrons from viewing gun 20. In particular, lines 104, 105 and 106 illustrate the transfer characteristics for potentials of +5, 9 and 10 volts, respectively, applied to the electroformed nickel mesh 28 of storage screen 25 as shown in the drawing. Transfer characteristic 104 which corresponds to a potential of +5 volts being applied to mesh 28 of storage screen 25 produces zero relative brightness when the storage surface potential is 6 volts. Likewise, transfer characteristics 105 and 106 which correspond to potentials of 9 and l0 volts, respectively, being applied to mesh 28 of storage screen 25, produce zero relative brightness for storage surface potentials of 4 and 3 volts. Of the transfer characteristics 104, 105 and 106, the transfer characteristic 105 is the more compatible with the other characteristics shown in FIGS. 4 and 5 to operate the storage device of FIG. 1 in a manner to produce half-tones as will be evident from the following explanation.

Referring now to FIG. 7, there is shown a chart of potentials on or associated with the storage screen 25 relative to the potential of viewing gun cathode 80 for the transfer characteristic 105 of FIG. 6. Thus, in accordance with the transfer characteristic 105, a potential of 9 volts is applied to electroformed nickel mesh 23 of storage screen 25. Also, there will be no positive potentials on the storage surface in that if and when produced, the ood electrons from viewing gun 20 will immediately commence to discharge them. As described in connection with transfer characteristic 105 of FIG. 6, the half-tone range of operation will exist between -4 volts and 0 volts on the storage surface, the 4 volts corresponding to zero brightness and the 0 volts corresponding to full brightness. This half-tone range is indicated by the shaded area 110 in the drawing. Upon initial energization of the tube, the storage surface of storage screen 25 will assume an initial potential of 9 volts relative to ground, which voltage corresponds to a zero volt potential drop across the storage layer 29 of storage screen 25. The storage surface may be charged in the positive direction by either the write or erase beams of write and erase guns 14, 15. As the storage surface is charged positive, however, the potential drop across storage layer 29 increases in the negative direction. Therefore, in the event that the erase beam is used, the writing speed is reduced to zero when the potential of the storage surface reaches 5.4 volts relative to the potential of viewing gun cathode as shown by characteristic 100 of FIG. 5. From the foregoing it is apparent that for the operating parameters previously specified, a 7 kilovolt erase beam has a stable point that is 5.4 volts relative to ground. Referring to FIG. 5, this may be explained by characteristic in terms of the operation of the erase beam. First, if the storage surface is at a potential that is positive with respect to 5.4 volts, that is, the potential thereon falls to the right of the 5.4 volt line of FIG. 7, the drop across the dielectric layer 29 of storage screen 25 is more negative than 3.6 volts and, hence, as indicated by characteristic 100, the storage surface will be discharged by the erase beam. On the other hand, if the storage surface potential is more negative than 5.4 volts, the potential drop across the dielectric layer 29 of storage screen 25 will be more positive than 3.6 volts and, hence, as shown by characteristic 100 of FIG. 5, the storage surface will be charged in a positive direction. As previously specified and in order to achieve the above results, it is necessary that the electron energy level of the erase beam be greater than the aforementioned predeterminable potential level and less than the second cross-over of the second-ary electron emission characteristic of the storage surface of storage screen 25. If the potential level of the erase beam were made greater than the second cross-over potential, super erasing may result making it necessary to bring the storage surface back to the operating ranges of potentials with the write beam. From the foregoing, it is apparent that when an erase beam having an energy level within the prescribed ranges is employed, the storage surface is selectively erased to a potential that is uniform within a fraction of a volt over the entire area thereof. One theory which may be used to explain this phenomenon is that the erase beam simultaneously charges the storage surface by secondary electron emission and discharges the surface by bombardment induced conductivity. That is, the storage surface must necessarily exhibit a secondary electron emission ratio greater than unity together with bombardment induced conductivity characteristics throughout respective ranges of electron energies incident thereon that overlap. Thus, in discharging the storage surface, the erase beam electrons impinge thereon with suticient energy to raise numerous electrons to the conduction energy level thus producing a corresponding number of holes which are attracted by the negative gradient to the mesh 28 of storage screen 25. A more pronounced negative gradient attracts more holes and, hence, discharges the storage surface more rapidly in a negative direction. A discharge in the negative direction reduces the gradient which, in turn, reduces the rate of the discharge by bombardment induced conductivity. For the above-mentioned conditions, there is zero net charging effect, i.e., the charging by secondary electron emission becomes equal to the discharging by bombardment induced conductivity, when the voltage drop across the layer 29 of storage screen 25 becomes equal to 3.6 volts in proceeding from the exposed surface of layer 29 to the electroformed nickel mesh 28.

As explained above, the erase beam produced by erase gun 15 is scanned over the storage surface of storage screen 25 thereby to charge the surface to a uniform potential of 5.4 volts relative to the potential of viewing gun cathode 80. Writing on the storage surface is then effected in a conventional manner by scanning the Write beam produced by write gun 14 over the storage surface by means of horizontal and vertical deflection voltage generators 56, 58. The intensity of the Write beam of write gun 14 may be adjusted so that a residual current corresponding to zero brightness of the signal applied to input terminal 53 corresponds to a minimum write beam current which is just suiiicient to charge the storage surface in a positive direction to i-4 volts relative to ground and to a maximum write beam current which charges the storage surface to zero volts relative to ground corresponding to full brightness thereby to produce a charge pattern on the storage screen 25. Then, as shown by the transfer characteristic 105 of FIG. 6, the flood electrous emanating from the viewing gun 20 penetrates through the interstices Within each elemental area of storage screen 25 in proportion to the charge thereon and proceed to the viewing screen 22 to produce a visual image `of the charge pattern. This charge pattern is selectively erased by the erase beam produced by erase gun 15 in the manner hereinbefore described. The horizontal and vertical deflection voltage generators 74, 76 which scan the erase beam over the storage screen 25, if desired, may be synchronized with the horizontal and vertical generators 56, 58 so as to erase the charge pattern from storage screen 25 immediately prior to when it is rewritten by the write beam of write gun 14.

Lastly, referring to FIG. l, the switch 69 may be connected to contact 71 so as to reduce the energy level of the beam produced yby erase gun 15 to of the order of 4.4 kilovolts. At this energy level, as shown by characteristic 94 of FIG. 4, the beam will not affect the charge pattern on storage screen 25. Thus, the erase beam may now be modulated by a signal applied to terminal 112, which signal is coupled through a capacitor 113 and across a load resistor 114 to the intensity grid 67 of erase gun 15 and scanned over the viewing screen 22 to produce a short persistence image thereon without affecting the charge pattern on storage screen 25. It is, of course, within the scope of the teachings of the present invention to utilize an entirely separate electron gun of appropriate energy level for writing directly on the viewing screen 22 without affecting the charge pattern on -storage screen 25. On the other hand, if the erase beam is not modulated with an intelligence signal in the manner described above and a charge pattern exists on the storage surface of storage screen 25, the secondary electrons collected by the collector grid 26 will be representative of the charge pattern. In flowing through the resistor 35, these secondary electrons from collector grid 26 develop an output signal available at -the capacitor 39 which output signal does not result in the destruction of the charge pattern which it represents.

What is claimed is:

1. A storage tube comprising a storage screen including a conductive substrate, a uniformly thin layer of cubic zinc sulfide disposed over at least a portion of said conductive substrate to provide storage surface; means for producing a negative voltage drop across said layer of cubic zinc sulfide; means for collecting secondary electrons ejected from said storage surface thereby to cause electrons of a predetermined energy level to charge said storage surface by secondary electron emission and to discharge said storage surface by bombardment induced conductivity at the same rates; means for producing an electron beam having a first energy level, said fir-st energy level being less than said predetermined energy level and within the range of potential levels wherein the secondary electron emission ratio of said layer of cubic zinc sulfide is greater than unity; means for scanning said electron beam of -said first energy level over said storage -screen thereby to charge elemental areas of said storage surface in positive directions in proportion to the intensity of said electron beam of said rst energy level incident thereon to produce a charge pattern; means for producing an electron beam of a second energy level, said second energy level being greater than said predetermined energy level and within the range of potential levels wherein the secondary electron emission ratio of said layer of cubic zinc sulfide is greater than unity; and means for scanning said storage screen with said electron beam of said second energy level thereby to induce conductivity through said layer of cubic zinc sulfide to said conductive substrate thereby to discharge said storage surface to a determinable reference potential level.

2. A storage tube comprising a storage screen including a conductive substrate, a uniformly thin layer of cubic zinc sulfide disposed over at least a portion of said conductive substrate to provide storage surface; means for producing a negative voltage drop across said layer of cubic zinc sulfide; means for collecting secondary electrons ejected from said storage surface thereby to cause electrons of a predetermined energy level to charge said storage surface by secondary electron emission and to discharge said storage surface by bombardment induced conductivity at the same rates; means for producing a charge pattern on said storage surface; means for producing an electron beam of said predetermined energy level; means for scanning said electron beam of said predetermined energy level over said storage screen whereby said charge pattern remains unaffected; and means responsive to said secondary electrons collected from said storage surface for generating a read-out signal representative of said charge pattern.

3. A half-tone visual display storage tube comprising a storage screen including a conductive screen and a uniformly thin layer of cubic zinc sulfide disposed over at least a portion of one side of said conductive screen to provide storage surface, said layer having a secondary electron emission ratio greater than unity and electron bombardment induced conductivity characteristics throughout overlapping ranges of electron energy levels; a viewing screen disposed adjacent to and coextensive with said storage screen on the side thereof opposite from said one side of said conductive screen; means for collecting secondary electrons ejected from said storage screen whereby electrons of a predetermined energy level charge and discharge said storage surface by secondary electron emission and bombardment induced conductivity, respectively at the same rates; first and second electron guns for producing first and second electron beams, respectively, said first and second electron beams being of elemental cross-sectional areas and having first and second energy levels within the range of potential levels wherein lsaid secondary electron emission ratio is grea-ter than unity, said first energy level being less than said predetermined energy level and said second energy level being greater than said predetermined energy level; means for modulating said first electron beam in accordance with an intelligence signal and for scanning said storage screen with said first modulated electron beam thereby to produce a charge pattern; means responsive to said charge pattern for producing a visual presentation thereof on said viewing screen; and means for scanning said storage screen with said second electron beam thereby to discharge said storage Surface to a uniform potential that is positive relative to said substantially fixed reference potential.

4. The half-tone visual display storage tube as defined in claim 3 wherein said conductive screen is a metal of the cubic lattice type.

5. The half-tone visual display storage tube as defined in claim 3 wherein a thin film of magnesium fluoride having a thickness of less than 1,000 Angstroms is disposed over said lstorage screen thereby to enhance the secondary electron emission characteristics of said storage surface. I

6. A half-tone visual display storage tube comprising a storage screen including a conductive mesh and a layer less than two microns thick of cubic zinc sulfide disposed over one side thereof; a viewing screen disposed adjacent to and coextensive with said storage screen on the side thereof opposite from said one side of said conductive mesh; means including a viewing gun having a cathode maintained at a substantially flxed reference potential for directing flood electrons uniformly over said storage screen; means for maintaining said conductive mesh at a predetermined potential level that is in the range of from 5 to l5 volts negative with respect to said reference potential; means for collecting secondary electrons ejected from said storage screen; means for producing and scanning said storage screen with a first electron beam of elemental cross-section area, said first electron beam being of an energy level to charge said storage surface to a stable potential level that is intermediate said reference potential and said predetermined potential level; and means for producing and scanning said storage screen with a second electron beam of elemental cross-section area, said second electron beam being of an energy level to charge said storage surface in positive directions proportional to the intensity thereof at the instant of impingcment thereon to produce a charge pattern whereby said flood electrons penetrate through said storage screen to said viewing screen in proportion to the charge on said storage surface to produce a visual presentation of said charge pattern.

7. In an electronic storage device, a storage target having a secondary electronic emission ratio greater than unity and bombardment induced conductivity characteristics throughout a common range of electron energy levels comprising a metallic substrate having an exposed cubictype lattice structure and a uniformly thin layer of cubic zinc sulfide disposed over at least a portion of said cubictype lattice structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,435,436 Fonda Feb. 3, 1948 2,776,371 Clogston Jan. 1, 1957 2,821,653 Dyer Jan. 28, 1958 2,856,559 Knoll Oct. 14, 1958 2,884,558 Smith Apr. 28, 1959 2,929,957 Kirkpatrick Mar. 22, 1960 2,967,969 Stocker Jan. 10, 1961 2,981,863 Schneeberger et al Apr. 25, 1961 FOREIGN PATENTS 798,400 Great Britain July 23, 1958 

1. A STORAGE TUBE COMPRISING A STORAGE SCREEN INCLUDING A CONDUCTIVE SUBSTRATE, A UNIFORMLY THIN LAYER OF CUBIC ZINC SULFIDE DISPOSED OVER AT LEAST A PORTION OF SAID CONDUCTIVE SUBSTRATE TO PROVIDE STORAGE SURFACE; MEANS FOR PRODUCING A NEGATIVE VOLTAGE DROP ACROSS SAID LAYER OF CUBIC ZINC SULFIDE; MEANS FOR COLLECTING SECONDARY ELECTRONS EJECTED FROM SAID STORAGE SURFACE THEREBY TO CAUSE ELECTRONS OF A PREDETERMINED ENERGY LEVEL TO CHARGE SAID STORAGE SURFACE BY SECONDARY ELECTRON EMISSION AND TO DISCHARGE SAID STORAGE SURFACE BY BOMBARDMENT INDUCED CONDUCTIVITY AT THE SAME RATES; MEANS FOR PRODUCING AN ELECTRON BEAM HAVING A FIRST ENERGY LEVEL, SAID FIRST ENERGY LEVEL BEING LESS THAN SAID PREDETERMINED ENERGY LEVEL AND WITHIN THE RANGE OF POTENTIAL LEVEL WHEREIN THE SECONDARY ELECTRON EMISSION RATIO OF SAID LAYER OF CUBIC ZINC SULFIDE IS GREATER THAN UNITY; MEANS FOR SCANNING SAID ELECTRON BEAM OF SAID FIRST ENERGY LEVEL OVER SAID STORAGE SCREEN THEREBY TO CHARGE ELEMENTAL AREAS OF SAID STORAGE SURFACE IN POSITIVE DIRECTIONS IN PROPORTION TO THE INTENSITY OF SAID ELECTRON BEAM OF SAID FIRST ENERGY LEVEL INCIDENT THEREON TO PRODUCE A CHARGE PATTERN; MEANS FOR PRODUCING AN ELECTRON BEAM OF A SECOND ENERGY LEVEL, SAID SECOND ENERGY LEVEL BEING GREATER THAN SAID PREDETERMINED ENERGY LEVEL AND WITHIN THE RANGE OF POTENTIAL LEVELS WHEREIN THE SECONDARY ELECTRON EMISSION RATIO OF SAID LAYER OF CUBIC ZINC SULFIDE IS GREATER THAN UNITY; AND MEANS FOR SCANNING SAID STORAGE SCREEN WITH SAID ELECTRON BEAM OF SAID SECOND ENERGY LEVEL THEREBY TO INDUCE CONDUCTIVITY THROUGH SAID LAYER OF CUBIC ZINC SULFIDE TO SAID CONDUCTIVE SUBSTRATE THEREBY TO DISCHARGE SAID STORAGE SURFACE TO A DETERMINABLE REFERENCE POTENTIAL LEVEL. 